A positive electrode paste, a positive electrode sheet and a preparation method thereof

Abstract

The present invention belongs to the technical fields of polymer materials and lithium batteries, and relates to a positive electrode paste, a positive electrode sheet and a preparation method thereof. Aiming at the technical problems in the prior art that when the amount of binder in the positive electrode paste of a lithium battery is small, the internal resistance is small, and although the cycle performance is good, the adhesion of the electrode sheet is poor; when the amount of binder is large, the internal resistance is large, although the adhesion is good, the cycle performance is poor, and the amount of transition metal catalyst is high. The present application provides a positive electrode paste and a preparation method thereof, which enhance the adhesion of the positive electrode paste coated on the substrate, reduce the amount of binder, reduce the amount of transition metal catalyst, effectively improve the agglomeration phenomenon of the binder in the paste, improve the dispersibility, and eliminate the influence of the volume change of the active material during the electrochemical process. The present application also provides a positive electrode sheet, and the internal resistance of the positive electrode sheet battery core is small, the number of cycles is high, and the service life is long.

Description

Technical Field

[0001] The present invention belongs to the technical fields of polymer materials and lithium batteries, and specifically relates to a positive electrode slurry, a positive electrode sheet and a preparation method thereof. Background Art

[0002] In the slurry used for battery electrode sheets, the dispersibility and uniformity of particulate active materials directly affect the movement of Li + between the two electrodes of the battery, thereby affecting the performance of the lithium-ion battery. An ideal electrode slurry should be a suspension with uniform dispersion and high stability. In the positive electrode slurry of existing power lithium batteries, polyvinylidene fluoride (PVDF) is generally used as a binder. Polyvinylidene fluoride is a semi-crystalline polymer, and its molecular chain is linear and has a certain degree of flexibility. After the homogenization and dissolution stirring are completed, the molecular chain is generally in a straight line shape, and at this time, the dispersion consistency of the slurry is the best. In the initial stage of the dissolution of polyvinylidene fluoride, due to the entanglement of the side chain molecular chains, the molecular chains of polyvinylidene fluoride generally show a clustered entanglement, and at this time, the consistency of the slurry is poor, and it is necessary to shear and stir for a long time to open the molecular chains and improve the consistency. In addition, the amount of polyvinylidene fluoride in the positive electrode slurry of lithium batteries directly determines the internal resistance of the finished battery. The less the amount of polyvinylidene fluoride used, the smaller the internal resistance and the better the cycle performance. On the contrary, the adhesiveness of the coated electrode sheet is poorer; the more the amount of polyvinylidene fluoride used, the larger the internal resistance and the worse the cycle performance. On the contrary, the adhesiveness of the coated electrode sheet is better; therefore, under the premise of ensuring the adhesiveness of the coated electrode sheet, the less the amount of glue used, the better. There are solutions in the prior art that completely consider not using polyvinylidene fluoride as a binder. For example, Chinese Patent Application Publication No. CN114335895A, with an application date of December 27, 2021, and a title of: A functional layer material for coating lithium-ion battery separators, discloses an aqueous functional layer material for coating lithium-ion battery separators, and this functional layer material does not contain PVDF components. Its main body is micron particles with an average particle size of 0.5 to 20 μm, and it is composed of a polymer with a glass transition temperature above 0 °C and below 150 °C and an electrolyte swelling rate above 10% and below 300%. Coating this functional layer material on the separator can ensure good adhesion to the separator material and can replace PVDF to a certain extent. However, for the material prepared by this solution, the maximum peel strength is only 23.7 mN / mm, and a large amount of styrene is contained in the formula, which will be released at high battery temperatures and cause harm to the human body. In summary, at the present stage, PVDF is still an irreplaceable binder for positive electrode battery slurries, so improving the dispersibility of PVDF is a problem that needs to be solved at present.

[0003] Publication No. of Chinese Invention Patent Application: CN113793938A, Application Date: August 27, 2021, Title: A PVDF Binder for the Positive Electrode of Lithium Batteries / Sodium Batteries / Potassium Batteries and Its Preparation Method. The disclosed binder includes a solvent, PVDF, polyethyleneimine, polytetrafluoroethylene, an inorganic conductive agent, and a compatibilizer; the compatibilizer is a copolymer of tetrafluoroethylene and vinylidene fluoride. This solution uses a ternary blend binder, which improves the binding performance of the binder by generating more hydrogen bonds and chemical cross-linking effects, inhibits the swelling of active substances during the charge and discharge of the battery, and enables the active substances to better adhere to the current collector. However, this solution adds polyethyleneimine and polytetrafluoroethylene, and PVDF, polyethyleneimine, and polytetrafluoroethylene form a ternary blend binder, with complex preparation steps. Summary of the Invention

[0004] 1. Technical Problems to be Solved by the Invention

[0005] In view of the technical problems in the prior art that when the amount of binder (such as polyvinylidene fluoride or polytetrafluoroethylene) in the positive electrode slurry of lithium batteries is small, the internal resistance is small, the cycling performance is good, but the adhesion of the electrode sheet is poor; when the amount of binder is large, the internal resistance is large, although the adhesion is good, the cycling performance is poor, and the amount of transition metal catalyst is high. This application provides a positive electrode slurry and its preparation method, which enhances the adhesion of the positive electrode slurry coated on the substrate, reduces the amount of binder, reduces the amount of transition metal catalyst, effectively improves the agglomeration phenomenon of the binder in the slurry, improves the dispersibility, and eliminates the influence of the volume change of active substances during the electrochemical process. This application also provides a positive electrode sheet, and the internal resistance of the positive electrode sheet cell is small, the number of cycles is high, and the service life is long.

[0006] 2. Technical Solutions

[0007] To achieve the above object, the technical solutions provided are as follows:

[0008] A positive electrode slurry of the present invention includes the following components and ratios:

[0009] 7 - 9 parts of positive electrode active material, 2 - 6 parts of binder, 1 - 4 parts of dispersant, 0.5 - 2 parts of conductive agent, 25 - 30 parts of organic solvent;

[0010] The dispersant is a amphiphilic block copolymer of 2-(N,N-dimethylamino)ethyl methacrylate - butyl acrylate.

[0011] Preferably, the binder is polyvinylidene difluoride or polytetrafluoroethylene.

[0012] The active groups in the structure of the amphiphilic block copolymer of 2-(N,N-dimethylamino)ethyl methacrylate and butyl acrylate (PDMAEMA-b-PBA) can form a cross-linked structure with binders (such as polyvinylidene fluoride or polytetrafluoroethylene), effectively improving the agglomeration phenomenon of the binder in the slurry, enhancing the dispersibility, and maintaining the binding strength of the electrode sheet, eliminating the influence of the volume change of the active material during the electrochemical process.

[0013] Furthermore, the molecular weight of the dispersant is 29700, the molecular weight distribution is 1.40, and the conversion rate is 85%. When the molecular weight is between 25000 and 30000, the best interaction with the binder and the cathode active material can be achieved. The narrow molecular weight distribution indicates that the molecular weight of the prepared dispersant is concentrated. A conversion rate >80% can enable large-scale production, and 85% is the best.

[0014] Furthermore, the cathode active material is lithium iron phosphate, lithium manganese iron phosphate or ternary material. Currently, the most widely used cathode materials for lithium-ion batteries are lithium iron phosphate and ternary materials. The binder in the system can be selected from PVDF or PTFE, which belong to hydrophobic polymer materials. While the dispersant of the present invention, the amphiphilic block copolymer of 2-(N,N-dimethylamino)ethyl methacrylate and butyl acrylate (PDMAEMA-b-PBA), belongs to an amphiphilic copolymer and plays a role of a dispersion bridge between the binder and the active material, achieving a better dispersion effect.

[0015] Furthermore, the conductive agent is a mixture of carbon nanotubes and graphene, which can enhance the conductivity of the cathode slurry.

[0016] Furthermore, the mass ratio of the carbon nanotubes to the graphene is 5:4.

[0017] Furthermore, the organic solvent is N-methylpyrrolidone, which is a good solvent for the binder and the dispersant.

[0018] A cathode sheet is prepared from the cathode slurry described above.

[0019] Preferably, the cathode slurry is uniformly coated on an aluminum foil with a thickness of 10 - 15 μm, dried at 100 - 140 °C, rolled, slit and die-cut to produce a cathode sheet.

[0020] Furthermore, the coating amount of the cathode slurry on the cathode sheet is 0.037 - 0.043 g / cm 2 .

[0021] A method for preparing a cathode slurry includes the following steps:

[0022] It includes the step of preparing a dispersion liquid: dissolving the dispersant in a solvent to obtain a dispersion liquid with a mass fraction of 60%.

[0023] It includes the step of preparing a composition: Mix 7-9 parts of a positive electrode active material, 0.5-2 parts of a conductive agent, and 2-6 parts of a binder evenly, then add 25-30 parts of an organic solvent and ball mill to obtain the composition;

[0024] It includes the step of preparing a positive electrode paste: Add a dispersion liquid containing 1-4 parts of a dispersant to the composition and continue ball milling to obtain the positive electrode paste.

[0025] Preferably, the rotation speed of the ball milling is 190 rpm, the first ball milling is for 30 min, and the second ball milling is for 20 min.

[0026] Furthermore, the solvent is butyl acetate.

[0027] 3. Beneficial effects

[0028] Adopting the technical solution provided by the present invention, compared with the existing well-known technologies, it has the following beneficial effects:

[0029] (1) A positive electrode paste of the present invention includes 7-9 parts of a positive electrode active material, 2-6 parts of a binder, 1-4 parts of a dispersant methylacrylic acid-2-(N,N-dimethylamino)ethyl ester-butyl acrylate amphiphilic block copolymer, 0.5-2 parts of a conductive agent, and 25-30 parts of an organic solvent. The addition of the dispersant PDMAEMA-b-PBA can interact with the binder (such as polyvinylidene fluoride or polytetrafluoroethylene), causing the active functional groups of the two to undergo a cross-linking reaction, forming a spatial network structure and winding the fluororesin, effectively improving the agglomeration phenomenon of the binder in the paste, thereby making the paste viscosity more stable, the consistency better, improving the dispersibility, enhancing the adhesiveness of the positive electrode paste coated on the substrate, reducing the amount of the binder used, and reducing the amount of the transition metal catalyst used.

[0030] (2) A positive electrode sheet of the present invention is prepared from the positive electrode paste. It reduces the internal resistance of the battery cell, increases the cycle of the battery cell, and finally improves the battery cycle life. And it can maintain the binding strength of the electrode sheet and eliminate the influence of the volume change of the active material during the electrochemical process.

[0031] (3) A method for preparing a positive electrode paste of the present invention, dissolving the dispersant in a solvent to obtain a dispersion with a mass fraction of 60%; then mixing 7-9 parts of positive electrode active material, 0.5-2 parts of conductive agent, and 2-6 parts of binder evenly, adding 25-30 parts of organic solvent, and ball milling. Finally, adding the dispersion containing 1-4 parts of dispersant to the composition and continuing ball milling to obtain the positive electrode paste. The addition of the dispersant PDMAEMA-b-PBA effectively shortens the stirring and preparation time of the paste, not only improving production efficiency, but also effectively avoiding excessive water absorption of the paste caused by too long paste production time and the reduction of the effective component of the binder during high-speed dispersion of the dispersing disk of the mixer, ensuring the bonding force of the binder. Description of the Drawings

[0032] Figure 1 Particle size distribution diagrams of positive electrode pastes prepared with different contents of dispersant;

[0033] Figure 2 Particle size distribution diagrams of positive electrode pastes with different PVDF contents.

[0034] Among them, 1 is the particle size of the paste prepared in Comparative Example 1; 2 is the particle size of the paste prepared in Example 2; 3 is the particle size of the paste prepared in Example 3; 4 is the particle size of the paste prepared in Example 4; 5 is the particle size of the paste prepared in Example 5; 6 is the particle size of the paste prepared in Example 6; 7 is the particle size of the paste prepared in Example 7. Detailed Embodiments

[0035] The present invention will be further described below in conjunction with specific embodiments.

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] The present invention synthesized a structurally controllable amphiphilic block copolymer of 2-(N,N-dimethylamino)ethyl methacrylate-butyl acrylate (PDMAEMA-b-PBA) by ARGET-ATRP, and studied the effects of the obtained copolymer on the dispersion of the positive electrode paste and the bonding performance of the positive electrode sheet. The specific preparation steps are as follows:

[0038] (1) At room temperature, 0.0069 g (0.03 mmol) of Me6TREN, 0.0264 g (0.15 mmol) of AA, 0.227 g (3 mmol) of EtBriB, 0.0023 g (0.01 mmol) of CuBr2, 15.72 g (100 mmol) of DMAEMA, and 4.7 g of toluene (30% of the mass of DMAEMA) were successively added to a 100 mL three-necked flask.

[0039] (2) After mixing evenly by magnetic stirring, the mixture was evacuated and filled with argon five times repeatedly, and then reacted under magnetic stirring in an oil bath at 30 °C. The monomer conversion was measured by gas chromatography during the reaction.

[0040] (3) The synthesis of the block copolymer was carried out by a one-step method. That is, after the polymerization reaction of the above formula was completed, 12.817 g (100 mmol) of BA was directly added, and then 0.023 g (0.1 mmol) of Me6TREN, 0.0176 g (0.1 mmol) of ascorbic acid, 0.0023 g (0.01 mmol) of CuBr2, and 8.5 g of toluene (30% of the mass of BA) were added. After evacuating and filling with argon five times repeatedly, the mixture was placed in a reaction at 60 °C. The monomer conversion was measured by gas chromatography. The product obtained after the reaction was slightly green. After diluting with ethyl acetate, it was passed through an alumina column once, and the solvent was evaporated under reduced pressure to obtain a colorless and transparent polymer, which was the target product.

[0041] The prepared dispersant was a amphiphilic block copolymer of 2-(N,N-dimethylamino)ethyl methacrylate-butyl acrylate (PDMAEMA-b-PBA), with a molecular weight of 29700, a molecular weight distribution of 1.40, and a conversion rate of 85%.

[0042] Reagents:

[0043] (1) Lithium iron phosphate was purchased from Jiangxi Zhili New Energy Technology Co., Ltd., with the model number ZLF-1.

[0044] (2) Ternary (lithium nickel cobalt manganese oxide) was purchased from Hunan Changyuan Lithium Co., Ltd., with the model number LY303.

[0045] (3) Carbon nanotubes were purchased from Cabot (China) Investment Co., Ltd., with the model number GCNTs5(N)31.

[0046] (4) Graphene was purchased from Harbin Wanxin Graphite Valley Technology Co., Ltd., with the model number GNLC-O5.

[0047] (5) N-methylpyrrolidone (NMP) was purchased from Hefei Yiquan New Energy Technology Co., Ltd.

[0048] Example 1

[0049] A positive electrode paste and its preparation method according to this embodiment include the following steps:

[0050] (1) Prepare a dispersion: Dissolve the dispersant PDMAEMA-b-PBA in the solvent butyl acetate to obtain a dispersion with a mass fraction of 60%.

[0051] (2) Prepare a composition: Mix 1.6 g of positive electrode active material (lithium iron phosphate or ternary), 0.2 g of conductive agent (carbon nanotubes and graphene are mixed according to a weight ratio of 5:4), 0.6 g of binder PVDF, and after mixing evenly, add 5.4 g of organic solvent NMP, and ball mill with the rotation speed set at 190 r / min and stir for 30 min to obtain a composition.

[0052] (3) Prepare a positive electrode paste: Add the dispersion containing 0.2 g of dispersant to the composition, continue to ball mill at a rotation speed of 190 r / min, and continue to stir for 20 min to obtain the positive electrode paste.

[0053] Further prepare a positive electrode sheet: Coat the above positive electrode paste on an aluminum foil with a thickness of 10 - 15 μm at a coating amount of 0.037 - 0.043 g / cm 2 , and after vacuum drying at 100 - 140 °C, perform rolling, slitting, and die cutting to make a positive electrode sheet.

[0054] The positive electrode sheet obtained in this embodiment has an adhesion of 29.37 N / m.

[0055] Example 2

[0056] A positive electrode paste and its preparation method according to this embodiment are basically the same as those in Example 1, except that the dispersion containing 0.4 g of dispersant is added to the composition.

[0057] For the positive electrode sheet obtained in this embodiment, the particle size distribution is shown in Figure 1 , and the adhesion is 32.26 N / m.

[0058] Example 3

[0059] A positive electrode paste and its preparation method according to this embodiment are basically the same as those in Example 1, except that the dispersion containing 0.6 g of dispersant is added to the composition.

[0060] For the positive electrode sheet obtained in this embodiment, the particle size distribution is shown in Figure 1 , and the adhesion is 35.93 N / m.

[0061] Example 4

[0062] A positive electrode paste and its preparation method according to this embodiment are basically the same as those in Example 1, except that the dispersion containing 0.8 g of dispersant is added to the composition.

[0063] The positive electrode sheet prepared in this example has a particle size distribution as shown in Figure 1 , and the adhesion force is 35.18 N / m.

[0064] Comparative Example 1

[0065] A positive electrode slurry and its preparation method in this comparative example are basically the same as those in Example 1, except that no dispersant is added.

[0066] The positive electrode sheet prepared in this comparative example has a particle size distribution as shown in Figure 1 , and the adhesion force is 28.75 N / m.

[0067] Table 1 Performance statistics of positive electrode sheets prepared with different contents of dispersant

[0068]

[0069]

[0070] It can be seen from Figure 1 that after adding the dispersant, the particle size of the slurry becomes smaller and the particle size distribution becomes narrower. Among them, the effect of Example 3 is the most obvious. This is mainly because the dispersant forms a sufficient three-dimensional space barrier layer on the surface of the lithium iron phosphate particles, and multiple adsorption points generate multi-point interactions simultaneously through hydrogen bonds, covalent bonds, and ionic bonds, enhancing the adhesion between the polymer and the solid particles, thus effectively avoiding the agglomeration between particles.

[0071] It can be seen from Table 1 that when the addition amount of the dispersant is 0.6 g, the adhesion force of the prepared positive electrode sheet is the largest, which is 35.93 N / m, and the adhesion performance is the best. This is because the addition of the dispersant PDMAEMA-b-PBA can interact with PVDF, causing cross-linking reactions between their active functional groups, forming a three-dimensional network structure and entangling the fluororesin, thereby making the slurry viscosity more stable, the consistency better, and enhancing the adhesion of the positive electrode slurry coated on the substrate.

[0072] Example 5

[0073] A positive electrode slurry and its preparation method in this example are basically the same as those in Example 3, except that 0.4 g of the binder PVDF is added to the composition.

[0074] The positive electrode sheet prepared in this example has a particle size distribution as shown in Figure 2 , and the adhesion force is 29.68 N / m..

[0075] Example 6

[0076] A positive electrode slurry and its preparation method in this example are basically the same as those in Example 3, except that 0.8 g of the binder PVDF is added to the composition.

[0077] The positive electrode sheet prepared in this example has a particle distribution as shown in Figure 2 , and the adhesion force is 35.91 N / m.

[0078] Example 7

[0079] A positive electrode slurry and its preparation method in this example are basically the same as those in Example 3. The difference is that 1.2 g of the binder PVDF is added to the composition.

[0080] The positive electrode sheet prepared in this example has a particle distribution as shown in Figure 2 , and the adhesion force is 33.25 N / m.

[0081] Table 2 Performance statistics of positive electrode sheets prepared with different contents of binder

[0082]

[0083]

[0084] It can be seen from Figure 2 that when the mass ratio of the binder PVDF to the dispersant is 1:1, the dispersion effect is the best. As the amount of PVDF decreases or increases, the change in particle size distribution is not obvious.

[0085] It can be seen from Table 2 that when the addition amount of the binder PVDF is less than 0.6 g, the adhesion force of the positive electrode sheet is 29.68 N / m. When the mass ratio of PVDF to the dispersant is 1:1, the adhesion effect is the best, and the adhesion force of the electrode sheet is 35.93 N / m. When the addition amount of PVDF is greater than 0.6 g, as the amount of PVDF increases, the adhesion force of the electrode sheet changes little.

[0086] Example 8

[0087] A positive electrode slurry and its preparation method in this example are basically the same as those in Example 3. The difference is that the PVDF binder is replaced with polytetrafluoroethylene (PTFE) and the addition amount remains unchanged. The adhesion force of the positive electrode sheet prepared in this comparative example is 34.83 N / m, indicating that the self-made dispersant has a better dispersion effect on PVDF and PTFE, and the adhesion force of the prepared electrode sheet is better than that of the ordinary positive electrode slurry system.

[0088] It can be seen from the change of the resistivity values of the diaphragm in the examples and comparative examples that when 0.2 g of the dispersant is added, the internal resistance of the positive electrode sheet changes little. When the addition amount of the dispersant is 0.6 g, the internal resistance of the positive electrode sheet decreases significantly, and the resistivity is 0.3763 Ω·m, indicating that the addition of the dispersant enhances the dispersion effect of the system, reduces the particle size of the active material, and reduces the internal resistance.

Claims

1. A positive electrode paste, characterized in that: It includes the following components and ratios: 7 - 9 parts of cathode active material, 2 - 6 parts of binder, 1 - 4 parts of dispersant, 0.5 - 2 parts of conductive agent, 25 - 30 parts of organic solvent; The dispersant is a amphiphilic block copolymer of 2-(N,N-dimethylamino)ethyl methacrylate - butyl acrylate; the molecular weight of the dispersant is 25000 - 30000, the molecular weight distribution is 1.40, and the conversion rate is 85%.

2. The cathode paste according to claim 1, characterized in that: The cathode active material is lithium iron phosphate, lithium manganese iron phosphate or ternary material.

3. The cathode paste according to claim 1, wherein: The conductive agent is a mixture of carbon nanotubes and graphene.

4. The cathode paste according to claim 3, wherein: The mass ratio of the carbon nanotubes to the graphene is 5:

4.

5. The cathode paste according to claim 1, wherein: The organic solvent is N-methylpyrrolidone.

6. A positive electrode sheet, characterized in that: It is prepared from the cathode paste according to any one of claims 1 - 5.

7. The cathode sheet according to claim 6, characterized in that: The coating amount of the positive electrode paste on the positive electrode sheet is 0.037~0.043 g / cm 2 .

8. A method for preparing a positive electrode paste, characterized in that: It includes the following steps: It includes the step of preparing a dispersion: dissolving the dispersant in a solvent to obtain a dispersion with a mass fraction of 60%; It includes the step of preparing a composition: mixing 7 - 9 parts of cathode active material, 0.5 - 2 parts of conductive agent, 2 - 6 parts of binder evenly, and then adding 25 - 30 parts of organic solvent and ball-milling to obtain a composition; It includes the step of preparing a cathode paste: adding the dispersion containing 1 - 4 parts of dispersant to the composition and continuing ball-milling to obtain the cathode paste; The dispersant is a amphiphilic block copolymer of 2-(N,N-dimethylamino)ethyl methacrylate - butyl acrylate; the molecular weight of the dispersant is 25000 - 30000, the molecular weight distribution is 1.40, and the conversion rate is 85%.

9. The preparation method of a positive electrode paste according to claim 8, characterized in that: The solvent is butyl acetate.

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